As is well known to those skilled in the related arts, one of the primary components of a cathode ray tube (CRT) display apparatus is the electron gun or guns. In particular, a visual display is created on a phosphorescent screen by a CRT by scanning one or more electron beams across the screen to illuminate picture elements (pixels) on the screen in a pattern to create a desired image. A monochrome CRT typically comprises a single electron gun. A color CRT typically comprises three electron guns corresponding to the colors red, green and blue. The combination of the three primary colors in different ratios on any given pixel can create any color in the visible spectrum.
Typically CRTs are manufactured in a clean room environment. Even the slightest contamination by dust or other particles in the neck assembly (which houses the electron gun or guns and the space through which the electron beam or beams travel from the gun to the screen) may significantly affect operation of the CRT. Particularly, if a dust particle or other particle is struck by an electron beam in the neck of a CRT, it typically would cause the CRT to arc, causing a current surge in the CRT. The particular lead or leads on which the current surge would occur depends on a number of factors, such as the particular electron beam which struck the particle. The surge, however, would most likely occur on the focus leads and/or ground leads of the electron guns. If the current surge is great enough, it can burn out one or more of the electronic components of the CRT.
Even if an arcing event does not destroy or harm electronic components, it can cause the calibration of the electron gun or guns to be changed. For instance, in a color monitor, the three electron guns are calibrated relative to each other to create the desired color balance. If these guns are calibrated while a dust particle is in the electron path of one of the guns, they will be calibrated relative to each other to certain values. If a dust particle which existed in the electron path when the initial calibrations were made is later zapped (burned by the electron beam), the initial calibration may no longer be valid. For instance, if the red electron gun was adjusted to 40.2 volts in order to create the desired color balance, and a dust particle in its electron beam path is zapped, the 40.2 volts adjustment may no longer be valid and the display may become skewed toward the red end of the color spectrum.
Arcing, leading to operational failure of the CRT or at least color skew, is one of the more common field failure mechanisms for CRTs.
Most CRT manufactures have quality assurance tests which test the CRT for many possible defects, including the existence of particles in the electron beam path, which cause or may cause arcing or other field failures. It is known that as the number of arcing events occurring during manufacturing and/or testing increases, the more likely the unit is to continue arcing in the field. Such behavior is presumed to be due to the existence of dust or other particles in the CRT which are zapped during manufacturing and/or testing. Excessive arcing during manufacturing or testing, therefore, suggests that the CRT was manufactured under less than perfectly clean conditions. This, in turn, suggests that additional dust particles exist in the neck assembly.
It is possible and even likely that dust particles existing in the neck assembly will may move during transportation, thus not causing arcing until the unit is sold and in operation in the field.
Accordingly, it is desirable to determine how often a CRT arcs during the manufacturing and/or quality assurance testing of the unit. Although, during manufacturing and testing, a CRT unit may typically be powered up and operating for approximately four hours (during which time arcing can occur), most of this time is spent outside of the presence of a person who can actually observe the arcing. For instance, during manufacturing, a CRT is typically "burned" in an aging tunnel for at least an hour. In the burning process, a CRT unit is powered up and operated in a high temperature environment in order to stabilize the components, particularly, the aperture grill. The aperture grill affects the alignment of the beams and typically does not stabilize until at least thirty to forty-five minutes of operation. The burning stage is normally conducted outside of the presence of the observation of humans.
U.S. patent application Ser. No. 08/519,511 filed on even date herewith by applicant, and incorporated herein by reference, discloses an apparatus for detecting and counting the number of times a CRT arcs. The disclosed apparatus comprises an inductor comprised of a split ferrite toroid core wound with ten turn wire which can be coupled around the leads of a CRT through which a current spike will run during arcing. Typically, a current spike caused by arcing will run through the focus leads and/or ground leads of the CRT. Accordingly, a probe should be coupled around each of those leads.
Each probe is coupled through level setting circuitry to a current pulse detector which triggers a monostable mode timer each time a current of a predetermined value is induced in the wire wound around the core of the probe. The level setting circuitry includes a variable resistor which is user adjustable to set the predetermined current level (i.e., the sensitivity of the arc counter). A current is induced in the wire wound around the core by the electromagnetic field created by a current spike in the lead around which the probe is positioned.
The output of the timer is fed to a counter which counts the number of arcing events. The counter, in turn, is coupled to a display unit for displaying the number of arcing events detected.
Each probe of the arc counter includes an adjustable resistor for setting a value of the predetermined current which will trigger the monostable mode timer (and thus the counter). This predetermined current value is dictated by the current which runs through the lead to which the probe is electromagnetically coupled. The predetermined current value is set to a value which is less than the current that would be caused to flow in the inductor wire by the current surge in the sensed CRT lead caused by an arcing event, but greater than the current that would be caused to flow in the inductor wire by the current flow in the sensed lead during normal operation of the CRT.
The reader is referred to the aforementioned U.S. application Ser. No. 08/519,511 for a more detailed description of the arc counter discussed above.
Once the desired predetermined current level is determined, the arc counter must be calibrated to it.
Accordingly, it is an object of the present invention to provide a method and apparatus by which an arc counter, such as the arc counter disclosed in U.S. application Ser. No. 08/519,511, can be calibrated.
More broadly, it is an object of the present invention to provide a method and apparatus by which any current detector can be calibrated.
It is a further object of the present invention to provide a method and apparatus by which an arc counter or other current detector can easily be calibrated to a predetermined trigger value with a small margin of error.